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“A vegetarian diet could be the key to a long life,” according to The Daily Telegraph. The newspaper says that extreme diets “just above malnutrition levels” might add an extra 25 years to UK life expectancy.

The news is based on a study investigating exactly what leads to the extended life and reduced fertility seen in flies fed a highly calorie-restricted diet. The study suggested that it was the low level of certain amino acids (building blocks of protein) in the diet that was responsible for the effects of the restricted diet.

It would be far more difficult to control a human diet in this way, and even if it were possible, the effects on lifespan and fertility might not be the same. Equally, this study on flies cannot tell us whether a vegetarian diet can increase lifespan in humans. We all need to eat foods that contain proteins, whether we are vegetarian or not. Individuals should aim for a balanced diet if they want to maintain good health.

Where did the story come from?

Dr Richard C Grandison and colleagues from University College London carried out this research. The study was funded by the Wellcome Trust and Research into Ageing. The study was published in the peer-reviewed scientific journal Nature.

What kind of research was this?

This was animal research looking at what aspects of an ultra-low calorie diet increase lifespan in the fruit fly (Drosophila). Previous research has shown that a restrictive diet can increase the lifespan of various microorganisms and animals, including primates. The diet improved health in older animals, but it also reduced fertility. The researchers wanted to investigate whether these effects were caused by the specific balance of nutrients rather than the calorie restriction. To do this, they looked at how adding different nutrients back into the restricted diet affected lifespan and reproduction in the flies.

Animals such as fruit flies play a useful role in scientific research because they do not normally live for very long. This allows researchers to assess the impact of different environments on lifespan in a short period. However, human lifestyles and lifespan are very different from those of flies. Therefore the findings in flies may not be applicable to humans.

The best way to test whether a vegetarian diet increases human lifespan would be a prospective cohort study following vegetarians and non-vegetarians over time to see how long they lived. Even this type of study would have design limitations, in that it would need to take into account other differences between vegetarians and non-vegetarians.

What did the research involve?

The researchers fed the flies an extremely low-calorie diet that contained just enough of each component to ensure they were not malnourished. They then tested whether reintroducing various nutrients into this restrictive diet affected lifespan and fertility (measured by the amount of eggs laid).

The nutrients added included vitamins, fats, carbohydrates and amino acids (the building blocks of proteins). The researchers also looked at the effects of adding either non-essential amino acids, which the body can make for itself, and the essential amino acids, which the body can only obtain through diet. They also looked at the effects of individual amino acids.

What were the basic results?

The researchers found that:

Adding amino acids to the ultra low-calorie diet decreased lifespan but increased the fertility of the flies (they laid more eggs).

Flies eating the ultra low-calorie diet plus added amino acids had a similar lifespan to flies that were fed a normal, full-calorie diet. No other added nutrients (vitamins, fats or carbohydrates) had this effect.

Non-essential amino acids slightly decreased lifespan but had no effect on fertility.

Adding the essential amino acids substantially decreased lifespan, reducing it by the same degree as a full feeding diet did. Essential amino acids also increased fertility.

The researchers found that the essential amino acid methionine was specifically responsible for increasing fertility back to normal levels. However, it did not affect lifespan. Adding other essential amino acids to the diet did not affect lifespan.

How did the researchers interpret the results?

The researchers conclude that the effect that a restricted diet had on lifespan and fertility in flies can be attributed to the low levels of essential amino acids it contains. They also say that in mammals a suitable balance of nutrients in the diet may cause the same extended lifespan offered by dietary restriction but without the inherent reduction in fertility that comes with it.

Conclusion

The Daily Telegraph acknowledges that this research was in fruit flies, but it overstates the implications that this research may have for humans. Fruit flies have a useful role in scientific research into lifespan but are obviously not representative of humans. These kinds of fly studies are generally performed as an initial step to inform future research, and to help investigate theories that are difficult to investigate in humans. For example, manipulating diet in this way in humans is unlikely to be feasible or ethical.

This study cannot tell us whether a vegetarian diet can increase lifespan in humans, as the newspaper reports have suggested. A far better way to test this claim would be by following human vegetarians and non-vegetarians over time to monitor their lifespan.